<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .outer{
            height: 500px;
            border-bottom: 10px black solid;
            margin: 50px auto;
            overflow: hidden;
        }
        .outer div{
            float: left;
            width: 100px;
            height: 100px;
            border-radius: 50%;
            background-color: #bfa;

            animation: ball .2s forwards infinite linear alternate;
        }
        div .box2{
            background-color: orange;
            animation-delay: .5s;
        }
        /* 创建小球下落的动画 */
        @keyframes ball{
            from{
                margin-top: 0;
            }
            to{
                margin-top: 400px;
            }
            /* 20%,60% ,to{
                margin-top: 400px;
                animation-timing-function: ease-in;
            }
            40%{
                margin-top: 100px;
            }
            80%{
                margin-top: 100px;
            } */
            /* to{
                margin-top: 400px;
            } */
        }
    </style>
</head>
<body>
    <div class="outer">
       <div class="box1">
       </div>
       <div class="box1">
       </div>
       <div class="box1">
       </div>
       <div class="box1">
       </div>
       <div class="box1">
       </div>
       <div class="box2">
    </div>
    </div>
</body>
</html>